12,675 research outputs found
The magnetic precursor of L1448-mm: Excitation differences between ion and neutral fluids
Shock modelling predicts an electron density enhancement within the magnetic
precursor of C-shocks. Previous observations of SiO, H13CO+, HN13C and H13CN
toward the young L1448-mm outflow showed an over-excitation of the ion fluid
that was attributed to an electron density enhancement in the precursor. We
re-visit this interpretation and test if it still holds when we consider
different source morphologies and kinetic temperatures for the observed
molecules, and also give some insight on the spatial extent of the electron
density enhancement around L1448-mm.
We estimate the opacities of H13CO+ and HN13C by observing the J=3\to2 lines
of rarer isotopologues to confirm that the emission is optically thin. To model
the excitation of the molecules, we use the large velocity gradient (LVG)
approximation with updated collisional coefficients to i) re- analyse the
observations toward the positions where the over-excitation of H13CO+ has
previously been observed [i.e. toward L1448- mm at offsets (0,0) and (0,-10)],
and ii) to investigate if the electron density enhancement is still required
for the cases of extended and compact emission, and for kinetic temperatures of
up to 400 K. We also report several lines of SiO, HN13C and H13CO+ toward new
positions around this outflow, to investigate the spatial extent of the
over-excitation of the ions in L1448-mm. From the isotopologue observations, we
find that the emission of H13CO+ and HN13C from the precursor is optically thin
if this emission is extended. Using the new collisional coefficients, an
electron density enhancement is still needed to explain the excitation of
H13CO+ for extended emission and for gas temperatures of\le 400 K toward
L1448-mm (0,-10), and possibly also toward L1448-mm (0,0). For compact emission
the data cannot be fitted. We do not find any evidence for the over-excitation
of the ion fluid toward the newly observed positions around L1448-mm.
The observed line emission of SiO, H13CO+ and HN13C toward L1448-mm (0,0) and
(0,-10) is consistent with an electron density enhancement in the precursor
component, if this emission is spatially extended. This is also true for the
case of high gas temperatures (\le400 K) toward the (0,-10) offset. The
electron density enhancement seems to be restricted to the southern, redshifted
lobe of the L1448-mm outflow. Interferometric images of the line emission of
these molecules are needed to confirm the spatial extent of the over-excitation
of the ions and thus, of the electron density enhancement in the magnetic
precursor of L1448-mm.Comment: Accepted for publication in A&A; 9 pages, 3 figure
A review of the use of information and communication technologies for dietary assessment
Presently used dietary-assessment methods often present difficulties for researchers and respondents, and misreporting errors are common. Methods using information and communication technologies (ICT) may improve quality and accuracy. The present paper presents a systematic literature review describing studies applying ICT to dietary assessment. Eligible papers published between January 1995 and February 2008 were classified into four assessment categories: computerised assessment; personal digital assistants (PDA); digital photography; smart cards. Computerised assessments comprise frequency questionnaires, 24 h recalls (24HR) and diet history assessments. Self-administered computerised assessments, which can include audio support, may reduce literacy problems, be translated and are useful for younger age groups, but less so for those unfamiliar with computers. Self-administered 24HR utilising computers yielded comparable results as standard methods, but needed supervision if used in children. Computer-assisted interviewer-administered recall results were similar to conventional recalls, and reduced inter-interviewer variability. PDA showed some advantages but did not reduce underreporting. Mobile phone meal photos did not improve PDA accuracy. Digital photography for assessing individual food intake in dining facilities was accurate for adults and children, although validity was slightly higher with direct visual observation. Smart cards in dining facilities were useful for measuring food choice but not total dietary intake. In conclusion, computerised assessments and PDA are promising, and could improve dietary assessment quality in some vulnerable groups and decrease researcher workload. Both still need comprehensive evaluation for micronutrient intake assessment. Further work is necessary for improving ICT tools in established and new methods and for their rigorous evaluatio
Variegate galaxy cluster gas content: Mean fraction, scatter, selection effects and covariance with X-ray luminosity
We use a cluster sample selected independently of the intracluster medium
content with reliable masses to measure the mean gas mass fraction and its
scatter, the biases of the X-ray selection on gas mass fraction, and the
covariance between the X-ray luminosity and gas mass. The sample is formed by
34 galaxy clusters in the nearby () Universe, mostly with
, and with masses calculated with the
caustic technique. First, we found that integrated gas density profiles have
similar shapes, extending earlier results based on subpopulations of clusters
such as those that are relaxed or X-ray bright for their mass. Second, the
X-ray unbiased selection of our sample allows us to unveil a variegate
population of clusters; the gas mass fraction shows a scatter of
dex, possibly indicating a quite variable amount of feedback from cluster to
cluster, which is larger than is found in previous samples targeting
subpopulations of galaxy clusters, such as relaxed or X-ray bright clusters.
The similarity of the gas density profiles induces an almost scatterless
relation between X-ray luminosity, gas mass, and halo mass, and modulates
selection effects in the halo gas mass fraction: gas-rich clusters are
preferentially included in X-ray selected samples. The almost scatterless
relation also fixes the relative scatters and slopes of the and
relations and makes core-excised X-ray luminosities and gas masses
fully covariant. Therefore, cosmological or astrophysical studies involving
X-ray or SZ selected samples need to account for both selection effects and
covariance of the studied quantities with X-ray luminosity/SZ strength.Comment: A&A, in press, minor language changes from previous versio
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Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.
Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes
Deep spectroscopic luminosity function of Abell 85: no evidence for a steep upturn of the faint-end slope
We present a new deep determination of the spectroscopic LF within the virial
radius of the nearby and massive Abell\,85 (A85) cluster down to the dwarf
regime (M* + 6) using VLT/VIMOS spectra for galaxies with m mag and mag arcsec. The
resulting LF from 438 cluster members is best modelled by a double Schechter
function due to the presence of a statistically significant upturn at the
faint-end. The amplitude of this upturn (),
however, is much smaller than that of the SDSS composite photometric cluster LF
by Popesso et al. 2006, -2. The faint-end slope of the LF in
A85 is consistent, within the uncertainties, with that of the field. The red
galaxy population dominates the LF at low luminosities, and is the main
responsible for the upturn. The fact that the slopes of the spectroscopic LFs
in the field and in a cluster as massive as A85 are similar suggests that the
cluster environment does not play a major role in determining the abundance of
low-mass galaxies.Comment: 6 pages, 4 figures, accepted at MNRAS lette
The role of low-mass star clusters in massive star formation. The Orion Case
To distinguish between the different theories proposed to explain massive
star formation, it is crucial to establish the distribution, the extinction,
and the density of low-mass stars in massive star-forming regions. We analyze
deep X-ray observations of the Orion massive star-forming region using the
Chandra Orion Ultradeep Project (COUP) catalog. We studied the stellar
distribution as a function of extinction, with cells of 0.03 pc x 0.03 pc, the
typical size of protostellar cores. We derived stellar density maps and
calculated cluster stellar densities. We found that low-mass stars cluster
toward the three massive star-forming regions: the Trapezium Cluster (TC), the
Orion Hot Core (OHC), and OMC1-S. We derived low-mass stellar densities of
10^{5} stars pc^{-3} in the TC and OMC1-S, and of 10^{6} stars pc^{-3} in the
OHC. The close association between the low-mass star clusters with massive star
cradles supports the role of these clusters in the formation of massive stars.
The X-ray observations show for the first time in the TC that low-mass stars
with intermediate extinction are clustered toward the position of the most
massive star, which is surrounded by a ring of non-extincted low-mass stars.
This 'envelope-core' structure is also supported by infrared and optical
observations. Our analysis suggests that at least two basic ingredients are
needed in massive star formation: the presence of dense gas and a cluster of
low-mass stars. The scenario that better explains our findings assumes high
fragmentation in the parental core, accretion at subcore scales that forms a
low-mass stellar cluster, and subsequent competitive accretion. Finally,
although coalescence does not seem a common mechanism for building up massive
stars, we show that a single stellar merger may have occurred in the evolution
of the OHC cluster, favored by the presence of disks, binaries, and gas
accretion.Comment: 17 pages, 11 figures, 3 Tables. Accepted for publication in A&
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